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Wind Terrain roughness factor calculation as per Eurocode 1991-1-4

Wind Terrain roughness factor calculation as per Eurocode 1991-1-4

Wind Terrain roughness factor calculation as per Eurocode 1991-1-4

Hello Forum,
I have doubt regarding wind load calculation as per EN 1991-1-4, roughness factor calculation,
Q.1) Equation 4.4 gives 2 formulas/criteria’s for the calculations of roughness factor which are
A)cr(z) = kr . ln(z/z0) for zmin ≤ z ≤ zmax
B)cr(z) = cr (zmin) for z ≤ zmin
Now consider value of z such that zmin ≤ z ≤ zmax, in this case am I supposed to calculate cr(z) only OR cr(z) & cr(zmin) both?
Also if I have to calculate the cr(z) for z ≤ zmin, ultimately I will have to use formula A) by substituting the value of z = zmin, so what is the purpose of formula B)?
Instead could there be only one formula such as A) cr(z) = kr . ln(z/z0)………for z ≤ zmax

Q.2) Just for a second assume that we have 2 peak pressure intensities qp(z) & qp(zmin) corresponding to z & zmin, how to use these values in calculation?
1) Apply qp(z) from ground level upto height z
2) Apply qp(zmin) from ground level upto height zmin and qp(z) for height zmin upto z
Please help me to clear my confusion.


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